As is known, there are two fundamental ramjet concepts. These comprise subsonic combustion and supersonic combustion, both of which are applicable to vehicles flying at supersonic speeds. In the subsonic combustion ramjet, the fuel is burned in air which has been slowed down to subsonic speeds within the engine. In a supersonic combustion ramjet, the fuel is burned in air which remains at supersonic velocities throughout the engine. Examples of supersonic combustion ramjet engines are shown, for example, in U.S. Pat. Nos. 3,280,565 and 3,430,445.
A subsonic combustion ramjet operates best at supersonic flight speeds and, therefore, must be boosted to ramjet ignition speed by a first stage, usually a solid rocket. Practical upper flight limits for a subsonic combustion ramjet are usually between 500 and 8000 feet per second. If higher flight speeds are desired with ramjet propulsion, it is necessary to change from a subsonic combustion ramjet to a supersonic combustion ramjet. However, using the same basic ramjet requires effective geometry changes to the engine. Past and current efforts to develop dual-mode (i.e., subsonic-supersonic combustion) ramjets have required complicated mechanical geometry changes and/or complicated fuel injection location control with compromised performance. The major problem is to convert the geometry of the engine from a double-throat to a single-throat configuration.